1. Field of the Invention
The present invention relates to a laser doppler velocity system for variable beam focusing, and more particularly, to a laser doppler velocity system for variable beam focusing that can easily change a focal length of laser beam in order to measure flow and shear stress, etc. flowing on the surroundings of a structure within flow field.
2. Description of the Related Art
With the recent development of optical measurement technology, a laser doppler velocimeter (LDV) system that is a laser doppler velocity system using doppler effect has been widely used in measuring flow-velocity of a flow field.
As a general measurement method for measuring flow-velocity, a probe mounted with a focusing lens is installed so that a focus thereof is formed on an external measurement position of a flow field mounted with a visible window and then scattered light of flow particles is detected by scanning laser thereon, thereby measuring flow-velocity. In order to move a measurement point, the probe is transferred mechanically using a transfer device, etc. The reason why the probe is transferred according to measurement points is that the focal length of the focusing lens is fixed.
Meanwhile, when measuring flow, flow-velocity information in a boundary layer region of a structure within the flow field is very important in view of engineering.
FIG. 1 is a schematic view illustrating velocity gradient of a boundary layer shown on a surface of a structure within a flow field. As shown in FIG. 1, from a surface 10 to a certain region 30, there is a boundary layer 30 having velocity gradient U(y) by receiving shear friction force of the surface. Within the boundary layer 30, as being closer to a structure by the shear friction force with the structure, flow-velocity is gradually reduced to be “0” on the surface 10 of the structure. On the position that is distant from the surface by the certain region or more, the effect of shear force is little to cause a region where flow-velocity U0 is the same. In particular, it has been known that in a low boundary layer 20 region, the velocity gradient is linear, and surface shear stress of the structure is linearly proportional to the velocity gradient in the case of Newtonian fluid. Therefore, the exact flow measurement within the boundary layer 30 in a y direction is very important in obtaining information on the surface shear stress as well as on the flow-velocity distribution around the structure.
As such a boundary layer is a region adjacent to the surface of the structure, and in order to obtain the surface shear stress, etc. sometimes requires securing a plurality of measurement points within 1 mm of the surface so that a LDV system having a fine resolution is needed, and as such there is a limit in using a general LDV system according to the prior art and a measuring method thereof.
A small-sized LDV system proper for such a usage has been recently developed to be widely used for various applications. For example, MSE in the United States has developed a mini-LDV by making a probe small using a semiconductor layer so that the mini-LDV has been used in measuring flow of a boundary layer. Also, in order to measure surface shear stress of a structure by being approached closer to the surface, a measurement device for shear stress only that is to be integrated on the surface of the structure has been also developed. Since the measurement devices have laser focusing lenses whose focal lengths are fixed, in the case of the mini-LDV, a mechanical transfer device should still be used and in the case when the mini-LDV uses a small-sized sensor integrated on the surface, the measurement position cannot but be fixed.
FIG. 2 shows a general LDV, in particular, a concept view of a small-sized LDV probe integrated with a small-sized diode laser having low output when a focal length of a focusing lens is not long. Reviewing the operation principle, if power 131 for generating laser is supplied to a LDV probe 100 sighted on a measurement position in a flow field, laser beam is generated by a laser which is a device in a light generating unit 130, and this is divided into parallel light 132 by a beam splitter 150 and a reflector 151, thereby being transferred to a focusing lens 110. The laser beam focused on a measurement point 133, that is, a focal length y, by the focusing lens forms a fringe and suspended particles 101 passing through the region scatter the laser beam.
The scattered light 142 causes a frequency shift (doppler effect) corresponding to the velocity of the suspended particles, and a portion of the scattered light 142 is returned to be focused by a light receiving lens 120 within the probe. The focused optical signals are transmitted to a frequency analyzer 141 through an optical cable 140. The doppler frequency is detected from the frequency analyzer 141, such that the flow-velocity at the measurement point can be known.
In a general laser focusing lens 110 of the system, the focal length y is fixed so that the entirety of the probe 100 should be moved in order to change the measurement point according to the y direction and thus, a separate mechanical transfer device should be coupled therewith. Therefore, an additional transfer device is required and thus, the volume of the device is increased, causing a limit in rapid and exact driving of the transfer of the probe.
Such a problem results from the reason that the probe itself should be transferred since the focal length of the focusing lens is fixed, as described above. Thereby, problems arise in that the rapid measurement of the flow-velocity is difficult and troublesome, and furthermore the efficiency of flow-velocity measurement is deteriorated.